This invention relates to an image forming apparatus, such as a digital copying machine or a laser printer, for forming an electrostatic latent image on the photosensitive drum by scanning and exposing the surface of the photosensitive drum with a single light beam or plural light beams generated by a semiconductor laser or lasers.
In recent years, various types of digital copying machines have been developed which form images by the scanning and exposing of a light beam and by electronic photographic processing. More recently, digital copying machines of the multi-beam type have been developed which generate plural light beams and cause the beams to scan an image in units of lines simultaneously to form an image at higher speed.
A digital copying machine of the multi-beam digital type comprises semiconductor laser oscillators for generating light beams, a polyhedral rotating mirror, such as a polygon mirror, for reflecting the light beam emitted from each of the laser oscillators toward the photosensitive drum and causing each light beam to scan the surface of the photosensitive drum, and an optical system unit serving as a light beam scanning apparatus mainly composed of a collimator lens and an f-xcex8 lens.
To produce images with high picture quality using a multi-beam optical system, light-beam power control is generally performed to equalize the optical power of each light beam on the photosensitive drum.
In the light-beam power control, a light sensing element senses the light beam directed onto the photosensitive drum and converts it into a current according to the amount of light. A current/voltage converter then converts the current outputted from the light sensing element into a voltage. An integrator then integrates the output voltage of the current/voltage converter. The output of the integrator is converted into a digital signal, thereby producing power sensing information. According to the power sensing information, the output power of the laser oscillators is controlled. As a result, the power of the light beams scanning the surface of the photosensitive drum is set at a specific value.
Furthermore, in the multi-beam type of digital copying machine, light-beam passing position control in the direction of feed is performed to form an image with high picture quality.
In the light-beam passing position control, the passing position in the feed direction perpendicular to the main scanning direction of the light beam directed onto the surface of the photosensitive drum is sensed. On the basis of the sensing result, the amount of deflection of the optical path necessary to set the passing position of the light beam at the desired position on the photosensitive drum is calculated. The optical path deflecting means, such as a galvanomirror, is controlled on the basis of the calculated amount of deflection of the optical path, thereby deflecting the passing position of the light beam on the photosensitive drum.
In the conventional light-beam power control, however, the power sensing characteristic may fluctuate seriously due to variations in the sensitivity of the photodiodes constituting the light sensing element or variations in the resistance values of the current/voltage resistors. For instance, the power sensing characteristic fluctuates significantly, when the sensitivity of the photodiodes is higher or lower than the specification value, or when the values of the resistance of the current/voltage conversion resistors are larger or smaller than the design values.
As described above, when the power sensing characteristic differs from one image forming apparatus to another because of such factors as the aforementioned variations, this may permit a difference in the density of the output image to occur between image forming apparatuses, which can degrade the image. To avoid the problem, the process conditions, including the condition for development, must be adjusted for each image forming apparatus.
Furthermore, in the conventional light-beam passing position control, when a certain laser oscillator or galvanomirror has malfunctioned, light-beam passing position control in the feed direction is not performed properly, degrading the picture quality seriously.
In such a case, a message, such as an error message or a serviceman call, is generally displayed and the use of the apparatus is stopped. When the image forming apparatus with the multi-beam optical system, which features high-speed recording, is left out of use (or in the down state), this gives the users a lot of trouble.
It is, accordingly, an object of the present invention to provide a light-beam scanning apparatus which features very small fluctuations in the power sensing characteristic of light-beam power sensing means.
Another object of the present invention is to provide an image forming apparatus capable of forming an image with high picture quality without any difference in the density of the output image between image forming apparatuses caused by variations in the light-beam power sensing characteristic in an image forming apparatus using a multi-beam optical system.
Still another object of the present invention is to provide an image forming apparatus capable of continuing to form an image without stopping until a serviceman has remedied the problem, when the light-beam generating means or optical-path deflecting means has malfunctioned in an image forming apparatus using a multi-beam optical system.
In order to achieve the above objects, according to one aspect of the present invention, there is provided a light-beam scanning apparatus comprising:
light beam generating means for generating a light beam and being able to change its output power; scanning means for scanning a scan surface with the light beam generated at the light beam generating means; light sensing means for sensing the light beam directed by the scanning means so that it may scan the scan surface and outputting a light sense signal proportional to the luminous energy of the light beam; light beam power sensing means which senses the power of the light beam on the basis of the light sense signal outputted from the light sensing means and includes adjusting means for adjusting more than one unit of the light beam scanning apparatus so that their power sensing characteristics may be equalized; and light beam power control means for controlling the power of the light beam generating means on the basis of the sensing result of the light beam power sensing means so that the power of the light beam scanning the scan surface may have a specific value.
The power of the light beam adjusted to a specific power by an optical power meter or the like is measured by the light beam power sensing means provided in the beam scanning apparatus. The measured value is displayed on the control panel. The value of the power sensing characteristic change variable resistor in the light beam power sensing means is adjusted so that the measured value may have a specific value. As a result, a light beam scanning apparatus with very small variations in the power sensing characteristic of the light beam power sensing means is provided.
According to another aspect of the present invention, there is provided an image forming apparatus which forms an image on an image retaining member by causing light beams to scan and expose the surface of the image retaining member, comprising: plural light beam generating means for generating light beams; scanning means for scanning the surface of the image retaining member with the light beams generated at the plural light beam generating means; light beam position sensing means for sensing the passing positions of the light beams in the direction perpendicular to the direction in which the light beams scan, the light beams being directed by the scanning means so that they may scan the surface of the image retaining member; calculation means for calculating the amount of deflection of the optical path to deflect the passing positions of the light beams to desired positions on the image retaining member on the basis of each sensing result of the light beam position sensing means; plural optical path deflection means which are provided for the light beams in a one-to-one ratio and are used to change each of the passing positions of the light beams on the image retaining member on the basis of the amount of deflection of each optical path calculated at the calculation means; malfunction sensing means for sensing that a malfunction has occurred in the plural light beam generating means; and control means for stopping the light-emitting operation of the light beam generating means that has malfunctioned, when the malfunction sensing means has sensed that a malfunction has occurred in the light beam generating means, and causing the image formation to be continued by the operation of the remaining good light beam generating means.
In the image forming apparatus using a multi-beam optical system, if a malfunction has occurred in the light beam generating means, such as semiconductor lasers, and the optical path deflection means, such as galvanomirrors, the light-emitting operation of the light beam generating means that has malfunctioned is stopped and the image formation is continued by the operation of the remaining good light beam generating means. Consequently, it is possible to provide an image forming apparatus capable of continuing to form an image without stopping until a serviceman has repaired the apparatus.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.